1. Field of the Invention
The present invention relates to a process of producing a multicolor optical element suitable for use in a liquid crystal display apparatus or the like to attain color display. More particularly, the present invention relates to a process of producing a multicolor optical element comprising a liquid crystal having a cholesteric regularity.
2. Related Art
Multicolor-indicating plates, color filters, polarizing elements, etc., have conventionally been known as multicolor optical elements comprising a liquid crystal having a cholesteric regularity. In the field of such multicolor optical elements, a method in which colored layers with predetermined areas and indication colors (e.g., red (R), green (G) and blue (B)) are photolithographically formed one by one as color-indicating regions, as described in Japanese Laid-Open Patent Publication No. 153798/1999, has been known as a method for obtaining the desired multicolored pattern. Japanese Laid-Open Patent Publication No. 83113/1984 describes such a method that the indication color of each color-indicating region is individually adjusted by controlling temperature.
However, the above method described in Japanese Laid-Open Patent Publication No. 153798/1999 is disadvantageous in that it requires increased production cost. This is because, in this method, it is necessary to repeat the photolithographic process the same number of times as the number of the indication colors of color-indicating regions required. The method described in Japanese Laid-Open Patent Publication No. 83113/1984 also has the following shortcomings: since the indication colors of color-indicating regions are adjusted by controlling temperature, the production process is inevitably complicated; and moreover, the boundaries between the color-indicating regions (color divisions) become unclear, making it difficult to create fine color-indicating regions.
In the meantime, International Laid-Open Patent Publication WO 00/34808 describes a method for forming a cholesteric layer (layer having a cholesteric regularity) bearing a predetermined multicolored pattern by applying active rays to a liquid crystal layer having a cholesteric regularity, containing, as its constituents, monomers having optically active groups so as to deactivate these optically active groups. Specifically, in this method, a cholesteric layer having color-indicating regions with predetermined areas and indication colors that have been created according to the active-rays-applied area and to the amount of active rays applied to the area is firstly made, and ultraviolet light having a wavelength at which the optically active groups are inactive is then applied to the cholesteric layer to fix the areas and indication colors of the color-indicating regions. According to this method, it becomes possible to obtain a cholesteric layer having a predetermined multicolored pattern in one photolithographic step if a photomask composed of regions having different transmittances is used to control the amount of active rays that reach to each color-indicating region to be created. Therefore, the production process is simplified; and in addition, it is easy to create fine color-indicating regions.
However, the above method described in International Laid-Open Patent Publication WO 00/34808 has such a drawback that: since the planar alignment of molecules in a cholesteric layer is readily disordered by the application of active rays, it is impossible to create color-indicating regions whose indication colors are excellent in intensity of color (color purity).
A method in which a cholesteric layer is subjected to a thermal alignment process at a temperature equal to or higher than the liquid crystalline phase transition temperature of a liquid crystal that constitutes the cholesteric layer has been known as a method usually used to align liquid crystalline molecules in a cholesteric layer (planar alignment). If this method is applied to a cholesteric layer bearing color-indicating regions created in a predetermined pattern by the application of active rays, the boundaries between the color-indicating regions (color divisions) become unclear due to thermal diffusion of the liquid crystalline molecules that occurs at the interface between each two color-indicating regions. It is thus difficult to obtain fine color-indicating regions by this method.
The present invention was accomplished in order to overcome the above-described shortcomings in the related art. An object of the present invention is therefore to provide a process of producing a multicolor optical element, by which a multicolor optical element containing fine color-indicating regions whose indication colors are excellent in intensity of color (color purity) can simply be produced at low cost.
The present invention provides, as a first embodiment, a process of producing a multicolor optical element, comprising the steps of: preparing a liquid crystal layer comprising a liquid crystal having a cholesteric regularity, the liquid crystal containing, as its constituents, monomers having optically active groups; applying active rays to the liquid crystal layer to deactivate the optically active groups, thereby forming a cholesteric layer having color-indicating regions with the desired areas and indication colors that have been created according to the active-rays-applied area and to the amount of active rays applied to the area; subjecting the cholesteric layer having the color-indicating regions to a thermal alignment process at a temperature lower than the liquid crystalline phase transition temperature of the liquid crystal; and three-dimensionally crosslinking this cholesteric layer for curing.
The present invention provides, as a second embodiment, a process of producing a multicolor optical element, comprising the steps of: preparing a liquid crystal layer comprising a liquid crystal having a cholesteric regularity, the liquid crystal containing, as its constituents, monomers having optically active groups; applying active rays to the liquid crystal layer to deactivate the optically active groups, thereby forming a cholesteric layer having color-indicating regions with the desired areas and indication colors that have been created according to the active-rays-applied area and to the amount of active rays applied to the area; partially three-dimensionally crosslinking the cholesteric layer having the color-indicating regions; subjecting the partially three-dimensionally crosslinked cholesteric layer to a thermal alignment process at a temperature equal to or higher than the liquid crystalline phase transition temperature of the liquid crystal; and three-dimensionally crosslinking this cholesteric layer for curing.
In the above-described first and second embodiments of the present invention, it is preferable that the processes further comprise, before the step of applying active rays to the liquid crystal layer, the step of subjecting the liquid crystal layer to a thermal alignment process at a temperature equal to or higher than the liquid crystalline phase transition temperature of the liquid crystal. It is also preferable that the cholesteric layer having color-indicating regions with the desired areas and indication colors that have been created according to the active-rays applied area and to the amount of active rays applied to the area be formed by applying active rays through a photomask composed of regions that have different transmittances and areas corresponding to color-indicating regions to be created. It is also preferable that the active rays be ultraviolet rays. In addition, the liquid crystal constituting the cholesteric layer is preferably a chiral nematic liquid crystal, and it is preferable that the cholesteric layer subjected to the thermal alignment process be three-dimensionally crosslinked and cured by the application of ultraviolet rays or electron beams.
According to the first embodiment of the present invention, active rays are applied to a liquid crystal layer comprising a liquid crystal having a cholesteric regularity, containing, as its constituents, monomers having optically active groups, thereby forming a cholesteric layer having color-indicating regions with the desired areas and indication colors that have been created according to the active-rays-applied area and to the amount of active rays applied to the area; and this cholesteric layer is then subjected to a thermal alignment process at a temperature lower than the liquid crystalline phase transition temperature of the liquid crystal that constitutes the cholesteric layer. It is therefore possible to effectively align the liquid crystalline molecules in the cholesteric layer (planar alignment) while effectively preventing thermal diffusion of the liquid crystalline molecules that occurs at the interface between each two color-indicating regions. A multicolor optical element containing fine color-indicating regions whose indication colors are excellent in intensity of color (color purity) can thus simply be produced at low cost.
According to the second embodiment of the present invention, active rays are applied to a liquid crystal layer comprising a liquid crystal having a cholesteric regularity, containing, as its constituents, monomers having optically active groups, thereby forming a cholesteric layer having color-indicating regions with the desired areas and indication colors that have been created according to the active-rays-applied area and to the amount of active rays applied to the area; this cholesteric layer is then partially three-dimensionally crosslinked; and the partially three-dimensionally crosslinked cholesteric layer is subjected to a thermal alignment process at a temperature equal to or higher than the liquid crystalline phase transition temperature of the liquid crystal that constitutes the cholesteric layer. It is therefore possible to effectively align the liquid crystalline molecules in the cholesteric layer (planar alignment) while effectively preventing thermal diffusion of the liquid crystalline molecules that occurs at the interface between each two color-indicating regions. A multicolor optical element containing fine color-indicating regions whose indication colors are excellent in intensity of color (color purity) can thus simply be produced at low cost.